There's not a specific reason for it, but the reason Any% No Telejumping is run is because it's the "classic" VVVVVV speedrun. 100% No Telejumping would not be significantly harder than 100% with telejumps, only longer. The reason for this is that you can't "cheese" a level like you can with Warp Zone and Tower in Any%, because you still have to get the trinkets, so the only time you'd lose from skipping Telejumps is from walking to the level.

VVVVVV